Regarding the phrase: accessibility = usability
I would say that accessibility is necessary but no always sufficient for
good usability. Usability is more subjective, while accessibility has been
defined as complying with the technical standards in WCAG or 508. And in
some cases, in the name of usability, accessibility is broken. For
example, many mouse only features are designed to make the software more
usable to the mouse capable users, but unfortunately have broken or did not
enabled the accessibility for the keyboard only user.
Regards,
Phill Jenkins